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Charcoal rot invaded soybean fields, resembles drought stress

Charcoal rot of soybeans is one of the few Indiana field crop diseases thriving in the extreme heat and exceptional drought, and it could reduce the yields that have otherwise survived the unusual weather.

Charcoal rot is caused by a fungus and infects seedlings early in the growing season. Symptoms, which mimic drought stress, aren't likely to appear until mid-season or later.

"Charcoal rot will be hard to diagnose in years like 2012, since it is difficult to distinguish it from symptoms of general drought stress," said Kiersten Wise, Purdue Extension plant pathologist. "Plants on hillsides or sandy areas will typically exhibit symptoms first."

While there are no in-season treatments to control charcoal rot, Wise said it's important that soybean growers scout for the disease and know which fields are susceptible.

Charcoal rot can build up in soils and can survive for several years.

"The fungus can infect a number of crops, including corn, which limits the effectiveness of tillage and rotations for managing disease," she said.

Growers should look for plants exhibiting symptoms similar to drought stress, such as wilting, yellowing and stunting of the plant. Because a large portion of soybeans are drought-stressed, Wise said growers need to pull some of those plants and split the lower stems to look for a gray discoloration, and dark, round fungal structures called microsclerotia.

"We need to know which fields have charcoal rot this year so we can manage it in future crops," she said. "Genetic resistance in soybean varieties is limited but may be available, so producers with confirmed fields of charcoal rot should work with seed dealers to select less-susceptible varieties. They also should avoid planting at high populations to reduce competition for water among plants."

Wise also said it's important to note that foliar fungicides are ineffective at preventing or reducing charcoal rot development.